In conventional cycloidal drives/transmissions generally used as speed reducers, a mobile gear with external teeth is propelled in a circular orbit by means of a crank of a high speed input shaft. Said mobile gear is meshed with a fixed annular gear with internal teeth. The crank radius is equal to the eccentricity between the two gears.
Typically, a coupling is used between the mobile gear and the low speed output shaft, capable of mechanically transmitting the rotational movement of said mobile gear, but not the orbital translation of the same. In said coupling the torque transmission is accomplished by means of a number of axial rods uniformly distributed on a disc integrated to the low speed shaft, which penetrate into an equal number of circular holes in the mobile gear.
Instead of using conventional spur gears in a cycloidal transmission, it is possible to use magnetic gears with magnets, such as is the case in the transmission described in Mexican patent application number MX/a/2012/001596 by the same applicant and titled Magnetic Cyclodial Transmission with Permanent Magnetic Gears for power transmission, and from which the present application claims priority. The use of magnetic gears is also described in other publications, such as U.S. Pat. Nos. 4,808,869, 4,850,821, 5,013,949 and 5,569,111. In some transmissions, permeable iron elements are used to guide a magnetic field of alternating direction resulting from the rotation of a central wheel with permanent magnets of alternating polarities causing a slow rotation of an exterior ring having a larger number of permanent magnets. Such is the case in the transmissions described in U.S. Pat. No. 3,378,710 and in U.S. Publication 2011/0127869 A1, and also in the transmission analyzed by P. O. Rasmussen et al in the article “Development of a high performance magnetic gear”, IEEE Transactions on Industry Applications, vol. 41, no. 3, 2005. In the paper “The cycloidal permanent magnetic gear”, IEEE Transactions on Industry Applications, vol.44, no. 6, 2008, by F. T. Jorgensen et al, a magnetic cycloidal transmission with a topology similar to that of the present invention is analyzed.
In the transmissions treated in all the previously mentioned references, the magnetic forces of attraction act across small gaps between the elements of the mechanism with no contact between them. In contrast with this characteristic, the magnetic gears of the present invention contact one another due to the fact that the mobile gear is free to move outwardly under the action of the magnetic attraction from the fixed gear and the centrifugal force, thus exerting pressure against the internal surface of said fixed gear as it rolls on it. In order for rolling to occur, the magnets of each gear must be assembled in such a way that they do not protrude from the contacting gear surfaces. In this way, the following advantages result: (1) the transmission's torque capacity is increased because, in addition to the magnetic force between the gears, a frictional force associated with the normal contact force, is developed; (2) the radial load on the bearing supporting the mobile gear is eliminated; (3) the need to accurately control the separation between the gears is eliminated and (4) the magnetic attraction forces between the gears are greater than would be if a gap were present.
In conventional cycloidal transmissions, a coupling is used between the mobile gear and the low speed shaft (the output shaft in the case of a speed reducer) that transmits only the rotational movement of that gear but filters out the circular translational movement resulting from its eccentric mounting. In such coupling, torque is transmitted by means of a number of axial pins uniformly distributed in a circular array in a disk integral with the low speed shaft, said pins penetrating in an equal number of circular holes in the orbiting gear, the radii of these holes being equal to the sum of the pin radii and the gear's axis eccentricity. In the present invention, roller bearings mounted on said pins are additionally incorporated to reduce the power loss due to friction between the pins and the surfaces of the holes of the orbiting gear. In this case, the outside radius of the bearings, instead of the pin radius, must be considered in calculating the radius of the holes. The incorporation of these bearings may not be feasible for large eccentricities.
One of the problems encountered in cycloidal transmissions, whether it be with spur gears or with magnetic gears, is the unbalance of the two gear system resulting in vibration due to the orbital movement of the mobile gear, given that its mass center is displaced in a circular trajectory which causes a centrifugal force of magnitude mω2r, where m represents the gear mass, ω the angular speed of the crank, and r the radius of the circular trajectory, said force represented by a vector which also rotates at a speed ω. A way to lower this vibration, as for example, in one embodiment of the apparatus of U.S. Pat. No. 4,567,790, is by incorporating a second mobile gear propelled by a crank at 180° to the first. The vectors which represent the centrifugal forces of both gears are of the same magnitude but in opposite directions, however they are not collinear as they occur in different transverse planes, giving rise to a moment on a rotating plane, reason by which the vibration is not completely suppressed.